Bipolar and field effect transistor integrated microcircuits are formed by a plurality of thin film depositions over a semiconductor substrate. Surface nonuniformities caused by the deposition of numerous thin films having different planar configurations create vertical excursions or steps in the top surface of the thin film device. Conductor buses are formed by first depositing or evaporating metal, such as aluminum, as a thin film on the top surface of the device and then photolithographically defining the conductors. The metal or aluminum lines must cover the vertical steps in the top surface of the device without a loss of continuity. One difficulty with such processes has been that the vertical steps in the top surface of the device cause shadowing effects during the evaporation or deposition of the metal thin film, reducing the thickness of the metal film in the vicinity of the vertical steps, leading to cracks or discontinuities in the metal conductor lines, which is a problem of long standing in the art.
Typically, production yield is significantly reduced by the poor metal step coverage, increasing the cost of producing such devices. One possible solution is to heat the devices so as to reflow, or at least partially reflow, the underlying dielectric film before metal deposition to prevent cracks or discontinuities in the metal layer. The disadvantage of such a solution is that if the device temperature is raised sufficiently to cause the dielectric film matter to at least partially melt, dopant impurities deposited in the underlying semiconductive substrate will significantly redistribute, which may destroy or at least impair the definition of the underlying transistors or devices formed in the substrate. Thus, it has been a long standing goal in the art to prevent discontinuities or cracks in the overlying metal layer without disturbing the electrical properties of the underlying semiconductive substrate.